A thickness vibration mode crystal resonator employing an AT-cut crystal vibrating element generally has an arrangement in which a pair of excitation electrodes is formed on front and rear surfaces of the crystal vibrating element, the excitation electrodes being exactly opposed to each other, and an alternating current is applied to the excitation electrodes. Various characteristics of such a piezoelectric resonator depend on the arrangement of the excitation electrodes. For example, when an electrode having a large size is used, the area of excitation can be increased, a series resonance resistance can be improved, and a frequency pulling range can be broadened.
Various characteristics of the crystal resonator also largely depend on the arrangement of the crystal vibrating element (crystal plate). For example, the plate surfaces of the crystal vibrating element may not be uniformly parallel to each other (not uniform plane parallelism) due to production conditions or production variations. In such a case, a spurious vibration is strongly excited, resulting in a deterioration in characteristics of the crystal resonator. Such a problem may be significant in a voltage control piezoelectric oscillator, which pulls a main vibration frequency by varying an external voltage, when the frequency is significantly pulled. Specifically, when the main vibration frequency is pulled, coupling with the spurious vibration is highly likely to occur, so that, disadvantageously, a frequency jump phenomenon occurs or oscillation is unstable.
FIG. 23 is a plan view showing a conventional surface mount crystal resonator before being hermetically enclosed. In FIG. 23, a crystal vibrating element 103 on which excitation electrodes 101 and 102 are formed is mounted in a package 10. In such an arrangement, when there is a variation in thickness of the crystal vibrating element, so that the plane parallelism is not perfect, a spurious vibration occurs.
FIGS. 24 and 25 are diagrams showing a state in which a spurious vibration is excited due to the plane parallelism of the plate thickness. FIG. 24(a) is a schematic cross-sectional view showing a state in which the excitation electrodes 101 and 102 are formed on an AT-cut crystal plate 7, where the plate thicknesses t1=t2, i.e., the plane parallelism of the plate surfaces is perfect. In such a crystal plate; frequency characteristics in which a spurious vibration does not appear in the vicinity of the main vibration are obtained as shown in FIG. 24(b). In FIGS. 24(b) and 25(b), the vertical axis represents impedances (Z) and the horizontal axis represents frequencies (FREQUENCY).
FIG. 25(a) is also a schematic cross-sectional view showing a state in which excitation electrodes 101 and 102 are provided on a crystal plate 7, where the plate thicknesses t1<t2, i.e., the plane parallelism of the plate surfaces is not perfect. In such a crystal plate, resonance characteristics in which spurious vibrations Sp appear in the vicinity of the main vibration are obtained as shown in FIG. 25(b). It is considered that such spurious vibrations Sp occur due to the imperfect plane parallelism of the plate surfaces. Specifically, it is known that, in a thickness-shear mode, the fs mode (symmetric mode) and the fa mode (oblique symmetric mode) are excited. In the oblique symmetric mode, vibration energy is canceled as a whole, so that the spurious vibration typically does not become manifest as a resonance peak. However, it is considered that, when vibration balance collapses due to the imbalance of the crystal plate, the mode becomes manifest as a spurious vibration.
Such a deterioration in characteristics of the crystal plate due to a variation in the plane parallelism, is disclosed in, for example, Patent Document 1 described below. In Patent Document 1, one of the opposed electrodes (excitation electrodes) is composed of two split electrodes. The split electrodes are caused to have substantially the same resonance frequency between the split electrodes and the other one of the opposed electrodes, thereby improving the characteristics. The split electrodes are electrically connected via a conductive means. In order to cause the split electrodes to have substantially the same resonance frequency, frequency adjustment is performed by, for example, subjecting either of the electrodes to vapor deposition or the like.
However, for the resonance frequency adjustment, either of the split electrodes is generally adjusted, but a step of determining a split electrode to be adjusted is required so as to adjust the vibration balance.
Also, in order to form such split electrodes, it is necessary to prepare a package having electrode pads that are electrically and mechanically connected separately to the respective electrodes of the crystal plate. Further, for example, it is necessary to form a wiring pattern commonly connected to the separate electrode pads, on a mount substrate, after frequency adjustment. Thus, it is troublesome to handle such an arrangement.
Further, the problem with the plane parallelism becomes manifest when the frequency is high. It is well known that the frequency of the AT-cut crystal plate, which is driven by a thickness vibration (e.g., a thickness-shear mode) is determined based on the thickness of the crystal plate, and the frequency is inversely proportional to the thickness. The deviation of the frequency per unit thickness increases with an increase in the frequency, so that the frequency adjustment of the crystal plate surface becomes more important.
For example, in the AT-cut crystal vibrating element, assuming that the fundamental frequency is 60 MHz, when the thickness is changed by 0.012 μm, the frequency deviation is 25 KHz. Assuming that the fundamental frequency is two times as high, i.e., 120 MHz, even when the thickness is similarly changed by 0.012 μm, the frequency deviation is four times as high, i.e., 100 KHz. As the frequency is further increased, the frequency deviation per unit thickness is increased.
Note that an arrangement in which the shape of the excitation electrode is varied is disclosed in Patent Document 2 described below, though the problem with the plane parallelism is not mentioned. Patent Document 2 discloses a crystal filter having an arrangement in which an input electrode and an output electrode are formed on one of the principal surfaces of a crystal plate and adjacent to each other with a predetermined interval, and a common electrode corresponding to the input and output electrodes is formed on the other principal surface. Basically, an arrangement that suppresses a non-harmonic overtone mode is disclosed.    Patent Document 1: JP 2001-196890 A    Patent Document 2: JP 10-98351 A